Suppression of spin-wave nonreciprocity due to interfacial Dzyaloshinskii-Moriya interaction by lateral confinement in magnetic nanostructures

Despite the huge recent interest towards chiral magnetism related to the interfacial Dzyaloshinskii-Moriya interaction (i-DMI) in layered systems, there is a lack of experimental data on the effect of i-DMI on the spin-wave eigenmodes of laterally confined nanostructures. Here, we exploit Brillouin light scattering (BLS) to analyze the spin-wave eigenmodes of noninteracting circular and elliptical dots, as well as of long stripes, patterned starting from a Pt(3.4-nm)/CoFeB(0.8-nm) bilayer, with lateral dimensions ranging from 100 to 400 nm. Our experimental results, corroborated by micromagnetic simulations based on the graphic processing units-accelerated mumax3 software package, provide evidence for a strong suppression of the frequency asymmetry Δf between counterpropagating spin waves (corresponding to either Stokes or anti-Stokes peaks in BLS spectra), when the lateral confinement is reduced from 400 to 100 nm, i.e., when it becomes lower than the light wavelength. Such an evolution reflects the modification of the spin-wave character from propagating to stationary and indicates that the BLS-based method of quantifying the i-DMI strength from the frequency difference of counterpropagating spin waves is not applicable in the case of magnetic elements with lateral dimension below about 400 nm.